This invention relates to a improvement and semitransparent diffractive elements and more particularly to a transparent and semitransparent type holograms and their making process. These diffractive elements are themselves transparent or semitransparent in visible (VIS) and/or near infrared (NIR) spectral region and yet are also endowed with the characteristics of a reflection type elements being observed under suitable angle. It means that reproduction in the transparent or semitransparent element of the present invention is effected only within specific reproduction angle range, while no hologram is recognised at other ordinary angles. This leads to the advantage that there is no visual obstruction of the article on which the diffractive element is laminated. FIG. 1 shows the basic constitution of the transparent or semitransparent diffractive element according to the present invention.
Demand for holograms has grown not only as the way of the record of sound or information but as the elements used in such activities of human beings as advertisement, security sector, safety technique, protection of product originality, money counterfeit protection etc. It is well known that one of the following replication technologies is usually used for mass production of any diffractive elements in suitable polymer materialsxe2x80x94hot embossing, injection moulding and casting.
Relief microstructure (master copy) is produced by one of the many high resolution fabrication technologies, the most commonly used being holographic exposure of suitable photosensitive material, including chalcogenides (U.S. Pat. No. 3,825,317), direct writing with focused laser and e-beam, optical photolithography with subsequent wet or dry etching.
In most cases, a nickel shim or stamper is electroformed or replica is produced through casting into epoxy resin. These replicas are used for own mass production of copies into polymers using injection moulding (CD fabrication), casting (production of gratings for spectrophotometers) or hot embossing, for example into transparent foil (M. T. Gale: J. of Imaging Science and Technology 41 (3) (1997) 211).
Transparent polymeric materials such as polyethylene with index of refraction n=1.5-1.54, polypropylene n=1.49, polystyrene, 1,6, polyvinyl chloride 1.52-1.55, polyester resin 1.52-1.57 etc. (for more examples see U.S. Pat. No. 4,856,857) or copolymers (for correction of index of refraction) can be used for transparent or semitransparent holograms and other diffractive elements production. Low refraction index value of these polymers or copolymers prepared from them determines their own reflectance (R about 4%), hence the holographic effect of diffractive structure developed in layers of these polymers is insufficient (U.S. Pat. No. 4,856,857). Under the term xe2x80x9cholographic effectxe2x80x9d used in the following text we will understand the phenomenon, that the hologram is very intensive in reflected light at suitable angle of observation. Low reflected intensity and thus the drawback of poor brightness of diffractive element recorded in the polymer layer is usually passed by forming a thin metallic film (generally Al) on the relief forming face of transparent polymeric layer (M. Miller: Holographyxe2x80x94theoretical and experimental fundamentals and their application, SNTL, Prague 1974 (in Czech); M. T. Gale: J. of Imaging Science and Technology 41 (3) (1997) 211).
Strong improvement of brightness achieved at the cost of loss of the transparency is the main drawback of such technique. Transparency or at least semitransparency of diffractive element is required or desired in many applications (for example protective diffractive elements on banknotes, identity cards with photo etc.). Some technical applications of diffractive elements are directly conditioned by transparency or semitransparency of created element (for example microlense array for CCD cameras, polarising filters etc.).
It is further known that to preserve (or to decrease only partly) the transparency of diffractive element and at the same time to improve holographic effect of the hologram recorded in the polymeric layer (further called layer 1), it is necessary to cover layer 1 by other transparent layer (further called layer 2) of different material (further called holographic effect enhancing material) which has in general different index of refraction n (i.e. higher or lower) than material of the transparent layer 1 (U.S. Pat. No. 4,856,857, U.S. Pat. No. 5,700,550, U.S. Pat. No. 5,300,764). The higher difference in index of refraction of polymeric bearing layer 1 and holographic effect-enhancing layer 2, the higher holographic effect can be achieved (U.S. Pat. No. 4,856,857).
It is as well known that very thin layer (with thickness to the limit 20 nm) of suitable metal (e.g. Cr, Te, Ge) can be used as such layer 2 deposited on the transparent 1 in which a hologram has been hot-formed. Such very thin metallic layer being used, relatively high transparency is preserved. Relatively strong enhancing of holographic effect can be achieved when the index of refraction of deposited metallic layer is either significantly lower (e.g. Ag n=0.8; Cu=0.7) or significantly higher (e.g. Cr n=3.3, Mn n=2.5, Te n=4.9) than index of refraction of transparent layer 1 (n about 1.5), (U.S. Pat. No. 4,856,857). Such thin metallic layers are deposited at transparent, diffractive element bearing layer 1 by vacuum deposition technique. The drawback of the application of thin metallic layer as holographic effect enhancing material is relatively high melting point of these materials and therefore difficult evaporating of many of these metals. An additional drawback is high absorption coefficient of metals. Already slight deviations in the thickness of evaporated metal layer implicate significant deviations in the transmissivity of the system (layer 1xe2x80x94bearing diffractive element+layer 2xe2x80x94metal) and moreover upper limit of the permissible thickness is very low (it depends on the metal, but in general it must not exceed 20 nm (U.S. Pat. No. 4,856,857)). According to our measurements evaporation of either 10 nm thick Cr layer or 4 nm thick Ge layer on the polymeric layer decreases its transmissivity down to about 30% (see FIG. 2).
In the present art, oxides of metals (e.g. ZnO, PbO, Fe2O3, La2O3, MgO etc.) halogenide materials (e.g. TlCl, CuBr, ClF3, ThF4 etc.) eventually more complex dielectric materials (e.g. KTa0.65NB0.35O3, Bi4(GeO4)3, RbH2AsO4 etc.) are used single or possibly in several layers deposited criss-cross as holographic effect enhancing layers (U.S. Pat. No. 4,856,857). The drawback of the application of these materials is the fact that their index of refraction values are very close to the index of refraction of transparent polymeric layer 1 (e.g. index of refraction values are 1.5 for ThF4, 1.5 for SiO2, 1.6 for Al2O3, 1.6 for RhH2AsO4 etc.) (U.S. Pat. No. 486,857). Accordingly, an amplification of holographic effect is relatively low. Many of these materials require again relatively high temperature for their evaporation and not least some of them are quite expensive or hardly preparable, what obstructs their mass application.
Further it is known, that binary chalcogenides of zinc and calcium as well as compounds Sb2S3 and PbTe (U.S. Pat. No. 4,856,857), eventually multilayer systems of these chalcogenides with oxides or halides (U.S. Pat. No. 5,700,550) or multilayer system ZnS and Na3AlF6 (U.S. Pat. No. 5,300,784) can be used as holographic effect enhancing. These materials are endowed with satisfactory index of refraction values (e.g. 3.0 for Sb2S3, 2.6 for ZnSe, 2.1 for ZnS). But short wavelength absorption edge of many of these materials (e.g. Sb2S3, CdSe, CdTe, ZnTe) lies within near IR region only and these materials are characterised by high values of absorption coefficient in VIS. Similarly with metal layer used as layer 2, only very thin layers of these materials can be used as holographic effect enhancing layer 2 to achieve at least semitransparency of final product. Transparency is again significantly influenced by thickness deviations. Additional significant drawback of these materials is their difficult vaporization (again similarity with metals) given by their high values of their melting points Tg alfaxe2x80x94ZnS 1700xc2x0 C., betaxe2x80x94ZnS 1020xc2x0 C., ZnSe greater than 1100xc2x0 C., ZnTe 1238xc2x0 C., CdS 1750xc2x0 C., CdSe greater than 1350xc2x0 C., CdTe 1121xc2x0 C., PbTe 917xc2x0 C.) Handbook of Chemistry and Physics 64th Edition 1983/84).
In the present art the process according to the scheme given in FIG. 3 is usually used in the mass production of transparent diffractive elements. Firstly a diffractive pattern is made in the layer 1, after it a thin dielectric or metallic layer is evaporated (perpendicularly or under specific incidence angle) a subsequently this evaporated layer is overlapped or laminated by another polymeric layer (M. T. Gale: Journal of Imaging Science and Technology 41 (3) (1997) 211). As above mentioned materials (metals, their oxides, halides, binary chalcogenides of Zn and Cd, Sb2S3 and PbTe) are used as layer 2 in the production of diffractive elements by this way, the method has the same drawbacks, e.g. high melting temperatures determine difficult deposition, even small deviations in the thickness cause large deviations in the transmissivity, comparable index of refraction of many of these materials with index of refraction of polymeric layer 1, eventually full non transparency in VIS.
Further it is known that holographic tape (relief phase holograms shaped in a vinyl tape) have improved scratch resistance being covered with such materials as waxes, polymers and inorganic compounds, besides others arsenic sulphide can be used (U.S. Pat. No. 3,703,407). In addition the coating enables tapes to be lubricated and enables tapes to be used in a liquid gate tape transport mechanism. In order to maintain the same diffraction efficiency as an uncoated tape, the minimum depth of this coating must be greater than the maximum peak-to-valley depth of any corrugation (U.S. Pat. No. 3,703,407).
The present invention does away with the drawbacks of the present-day techniques of transparent and semitransparent diffractive elements production.
Transparent and semitransparent diffractive elements, particularly holograms, consisting at least of two layers with a different index of refraction, whereof a first bearing layer (1) is a transparent polymer or copolymer having index of refraction lower than 1.7 and on said first bearing layer is deposited a second to exposure sensitive holographic effect enhancing high refraction index layer (2) constituted by substances based on chalcogenides with an index of refraction higher than 1.7 and a melting temperature lower than 900xc2x0 C., characterized in that the first diffractive pattern is mechanically shaped in the bearing layer (1) and/or in the the high refraction index layer (2) and at least one further second diffractive pattern is formed in the high refraction index layer (2), constituted by substances based on chalcogenides comprising at least one of the elements from the group sulphur, selenium, tellurium, the said chalcogenide based substances being selected from the group of binary, ternary and even more complex chalcogenide and/or chalcohalogenide systems, containing, in addition to S or Se or Te, as a more electropositive element some of the elements Cu, Ag, Au, Hg, B, Al, Ga, In, Tl, Si, Ge, Sn, Pb, N, P, As, Sb, Bi.
Chalcogenides based matters can contain further transient metal and/or at least one rare earth element, e.g. Pr, Eu, Dy.
Transparent or semitransparent diffractive element can further consists of other layers e.g. protective layer, adhesive layer, fragile layer, anchor layer. Protective layer protects layer 2 or layer 1 against environmental effect or against undesirable effect of consecutive exposure by UV light and improves resistivity of the final product. The layer can either be permanent part of the hologram or of the diffractive element or can be removable. Adhesive layer allows unrepeatable or repeatable anchoring of the hologram or other diffractive element on protected article, printed document etc. The function of fragile layer is to adhere the upper layer and the lower layer and yet effect destruction of diffractive element during peeling for the purpose of forgery. Anchor layer is used to improve adhesivity of diffractive element to the base supporting sheet or to releasable sheet in the case of application as seal, sticker, label etc.
Transparent layer 1 can be inseparable part of some larger product, in such case the high refraction index layer 2 can be for example sprayed on the layer 1.
Procedure of transparent diffractive elements production consists of the formation of the first diffractive pattern in a bearing layer 1 and subsequent deposition of high refraction index layer 2, which is formed with a chalcogenide based matters of different compositions. The deposition of different chalcogenide based matters can be consecutive or simultaneous and then the second diffractive patterns are formed in the said exposure-sensitive high refraction index layer 2.
Alternative way of transparent and semitransparent diffractive elements production is firstly to deposit high refraction index layer 2 on bearing layer 1 and only after that to originate a required the first diffractive pattern into high refraction index layers at elevated temperature for example using hot embossing technique. If the depth of diffractive pattern is greater than the thickness of high refraction index layer 2 (very common situation), practically identical product (FIG. 1) is obtained as when the previous procedure is used. If the embossing depths are lower than thickness of high refraction index layer, the layer 1 operates as carrier of high refraction index layer 2 only. After that the second diffractive patterns are formed in the said to exposure sensitive high refraction index layer 2.
High refraction index layer can be deposited on a previously coloured layer 1 and thus through the combustion of their colours (colour of layer 2 depending on the composition and thickness used) a required colour effect of transparent or semitransparent diffractive element can be achieved.
High refraction index layer 2 can be deposited either at low pressure e.g. using vacuum evaporation, sputtering or chemical vapour deposition (CVD) technique or at normal pressure as solution of chalcogenide based matters using e.g. spraying, painting or spin coating method.
The composition of high refraction index layer 2 formed with some chalcogenide based matters can be modified by exposure or annealing induced diffusion of metals and/or by halogens and/or oxygen, which are implanted into layer 2 by interaction of the layer 2 with halogen vapours or oxygen or by air hydrolysis.
The sensation of the first diffractive pattern shaped mechanically in layer 1 and/or layer 2 is modified by second diffractive pattern formed in layer 2 by exposure and/or annealing and/or by selective etching.
Exposure with radiation of suitable wavelength and intensity (values depend on the particular composition of high refraction index layer (2), e-beam, ions, X-ray radiation etc.) or annealing originates structural changes in high refraction index layer or it originates even changes in its chemical composition (e.g. diffusion of metal, which is in direct contact with high refraction index layer, hydrolysis, oxidation). Thereby a change of the value of index of refraction of layer 2 takes place (it usually increases) and thus the difference between values of index of refraction of bearing layer 1 and high refraction index layer 2 is modified. It results in a different optical perception of the product. A chemical reaction induced by exposure or by annealing, e.g. with surrounding atmosphere, can result in the transformation of chalcogenide material into fully different compound (e.g. oxide), the product of such reaction must again satisfy the condition, that its index of refraction is higher than 1.7.
Local exposure through the mask or holographic exposure or local annealing can produce a record of the further second diffractive pattern into the high refraction index layer 2; the record can be either amplitude (based on different absorption coefficient of exposed and unexposed part of layer 2) or phase type based on either different values of index of refraction of exposed and unexposed parts of layer 2 or based on different thickness of exposed and unexposed parts of the layer 2 (different thickness can be achieved not only directly during exposure but also by consecutive etching of layer 2 by using well-known methods); even here can be used the phenomenon of local photoinduced diffusion, hydrolysis, oxidation etc. and the matter of high refraction index layer 2 can, in the place of local exposure or annealing change its chemical composition; resulting record in the high refraction index layer 2 can partly modify visual perception of the hologram and in addition it can be seen in view-through.
As index of refraction values of majority of chalcogenides exceed the value n=2, application of chalcogenides layers as holographic effect enhancing layer 2 deposited on the transparent polymeric layers 1 with n less than 1.7 results generally in a significant visual perception. The transparency of final hologram or other diffractive element can be influenced through the thickness of layer 2.
Another important advantage of chalcogenide materials is the fact, that they can be synthesised in many systems in amorphous state and their glass forming regions are relatively wide. Being amorphous, these materials have not only very low scattering losses, but the possibility to prepare even nonstochiometric compounds takes place. Gradual mutual substitution of elements (not only S, Se and Te) in the composition of amorphous chalcogenides causes continuous changes in their index of refraction and reflectivity. Thus enhancement of holographic effect can be xe2x80x9ctailoredxe2x80x9d.
As a result of gradual mutual substitution of elements in the composition of amorphous chalcogenides arises gradual changes of their optical gap Egopt values (e.g. As40S60Ehd gopt=2.37 eV, As40S40Se20 2.07 eV, As40Se60 1.8 eV) followed by gradual changes in the position of short wavelength absorption edge. Thus the colour (for given thickness) of layer 2 can be changed as well and transparent and semitransparent systems of different colours endowed with high holographic effect can be produced. So even colourless polymeric layers 1 can be used for production of transparent or semitransparent diffractive elements of required colour using one (or more) chalcogenide based layer of suitable composition as a layer 2. Thus composition and thickness of chalcogenide layer 2 influence significantly the transparency of final product (hologram) (FIG. 4) and reflectivity (FIG. 5) and thus intensity of holographic perception (it increases with the reflectivity of layer 2).
Amorphous chalcogenides are mainly as thin layers photosensitive to exposure with radiation of suitable intensity and wavelength (given by composition of the layer), e-beam, ions etc. This property enables us to provide a supplementary correction of index of refraction, reflectivity and transmissivity of high refraction index thin layer using exposure induced structural changes (FIG. 6), by exposure induced reaction of photosensitive chalcogenide layer with metal (e.g. Ag) (FIG. 6) or with gas (O2, air humidity) induced transformation into different chemical substance, which must satisfy the condition that n greater than 1.7. Similar effects can be achieved by annealing.
If exposure or annealing are local only, procedures mentioned in the previous paragraph can result in the formation of an image (including holographic one) in the high refraction index layer, which can partly modify visual perception of the hologram and in addition it can be seen in view-through. Sectional views of structures developed using photoinduced structural changes and photoinduced metal diffusion are presented in FIGS. 7 and 8.
Further advantage of above mentioned chalcogenides are their low melting temperatures (usually 100-300xc2x0 C.). They can be therefore deposited by worldwide commonly used vacuum evaporation method. As the values of absorption coefficient in the region behind short wavelength absorption edge are low, even possible small deviation in the thickness influences much less the holographic effect enhancing than when thin metallic layers are used. Large areas of chalcogenide layers can be formed relatively easily using corresponding vacuum evaporation equipment. The thickness of the chalcogenide layer 2 can be adjusted by synchronising the evaporation rate with the feed speed of transparent bearing layer 1.
Further advantage of amorphous chalcogenides is the fact, that mass production of chalcogenides of many compositions exist worldwide and they are thus immediately commercially available at affordable price.